In hydraulically actuated devices, e.g. servo-mechanisms, clutches and brakes, it is a common practice to provide a fluid-displacement device in the form of a cylinder in which a piston is displaceable to generate a fluid pressure which is transmitted by an outlet port to the controlled device.
In a brake system, for example, this cylinder is referred to as a master cylinder and generally communicates at one end with a brake line and laterally via a reservoir port with a chamber, i.e. the brake fluid reservoir, adapted to receive fluid from the chamber when the piston is retracted beyond this latter port which is closed off from the cylinder upon an actuating displacement of the piston.
It is known to provide, in the region of this port, a recess or the like which can provide temporary communication between the chamber of the cylinder on the leading side of the piston seal or cup, and a region behind it at least during the start of the stroke of the piston.
Such a system, whose recess flanks rise with a shallow slope toward the deep portion of the recess, is described, for example, in U.S. Pat. No. 3,473,330, and German patent document No. 1,655,308.
The principles described are applicable not only to brake master cylinders, but also to other hydraulic actuators such as the cylinders used for actuating clutches, servo-mechanisms and the like.
While this master cylinder, or more generally actuating cylinder, was found to be highly effective for the purposes described in these publications, widespread use was prevented by the difficulty of forming the recess with flanks of shallow slope as there-described.
Specifically, the master cylinder could not be formed by a pressure casting (die casting) or injection molding process without expensive subsequent machining, or with inexpensive mold and techniques.
In many cases, therefore, while the principles could be used with considerable effect, the prohibitive fabrication costs and complex preparation process precluded such use.